yes, Mona distract them but she is never the real a. Ali was always the real -A. Mona only know alls of the information because she has Ali's diary from the Ali shrine.
Nobody can really tell because the shadows surrounding her lips confuse many people. There are many books about this subject debating on whether shes smiling or not. Like I said nobody really knows,...
Because she read Alison's old diary and discovered all of the Pretty Little Liars's secrets and information. Plus, she's evil and she wanted to haunt and scare the PLL.
In the book, Pretty Little Liars, when character, Mona, dies, the new "A" is really Alison. It's pretty complicated how it all works so it's best to finish reading the series to get more...